Conventional molded products of thermoplastic resin for applications such as electric appliances and OA equipment used at home, in offices, in plants, and the like mostly contain flammable thermoplastic resin. Therefore, molding materials of these products are generally required to contain a flame retardant for improving flame retardance of resin products.
Although many flame retardants have incombustibility, once combustion starts, they can hardly inhibit the spread of the combustion because thermoplastic resin becomes liquid and then drips with burning. Concerning incombustibility, for example, Underwriters' Laboratories (hereinafter abbreviated as UL) provide classes of Fire Test in standard 94, which include a class of high incombustibility (V-1, V-0) with anti-dripping property. In some cases, a polytetrafluoroethylene (PTFE) powder or an aqueous dispersion thereof along with a flame retardant are added to and melt mixed with a flammable thermoplastic resin so as to inhibit dripping and further improve safety.
PTFE is excellent in heat resistance, chemical resistance, and electrical insulation, and has specific surface characteristics such as water and oil repellency, anti-adhesion, and self-lubricating property. With these advantageous properties, PTFE is widely used for coating agents. Since PTFE is highly crystalline and has low intermolecular force, it can be made into fibers with a little stress. In addition, PTFE improves formability, mechanical properties, and the like when added to a thermoplastic resin. Thus, PTFE is also used for additives for thermoplastic resins.
For example, Patent Literature 1 discloses a compound which contains 0.1 to 5% (% by weight; the same shall apply hereinafter) of PTFE with a flammable thermoplastic resin, a flame retardant, and an incombustible fiber; Patent Literature 2 discloses a compound which contains a flame retardant and PTFE with a polyphenylene ether resin or styrene resin; Patent Literature 3 discloses a compound which contains 0.01 to 10% of a flame retardant of an organic alkali metal salt and/or organic alkaline earth metal salt and 0.01 to 2.0% of PTFE (ASTM D-1457, Type III) with an aromatic polycarbonate; and Patent Literature 4 discloses a compound which contains a flame retardant and PTFE with an acrylonitrile-butadiene-styrene copolymer (ABS).
As examples of polymer alloys of an aromatic polycarbonate and styrene resin, Patent Literature 5 discloses a compound which contains PTFE, an organobromine compound, and a flame retardant such as an antimony compound or a bismuth compound; Patent Literatures 6 to 9 disclose compounds which contain PTFE with a phosphorous compound; and Patent Literature 10 discloses a compound which contains PTFE with an alkali metal salt of an organic or inorganic acid. As an example of polyamides, Patent Literature 11 discloses a compound which contains PTFE with a flame retardant such as a phosphate ester. Patent Literatures 12 and 13 disclose resin compounds which contains polytetrafluoroethylene with polyolefin. Patent Literature 14 discloses a method of producing a polyolefin resin compound by mixing polytetrafluoroethylene and dispersion medium powder with a high shear force, making polytetrafluoroethylene into fibers, and then mixing the resulting fibers with polyolefin.
Examples of PTFE for anti-drip applications include a powder (generally called as a PTFE fine powder, standard specific gravity (SSG): 2.14 to 2.23, classified to ASTM D-1457, Type III) produced by coagulation and drying from a latex which is prepared by emulsion polymerization of tetrafluoroethylene (TFE); and an aqueous dispersion (generally called as PTFE dispersion) produced by adding a surfactant to the latex, and condensing and stabilizing the mixture.
The anti-drip function relies on PTFE particles of having tendency to fibrillate. Specifically, when a thermoplastic resin in a molten state is mixed with the fine powder or the dispersion, PTFE fine particles fibrillate by a shear force of mixing, and the fibrils are dispersed in the thermoplastic resin. The dispersed fibrils remain in final molded products of the thermoplastic resin, which inhibits dripping during combustion.
Before melt mixing of the flammable thermoplastic resin and PTFE, the flammable thermoplastic resin, which is in a state of powder, pellet, or liquid and the fine powder or the dispersion need to be premixed. The fine powder, however, has inferior handling characteristics because it tends to fibrillate even at normal temperature and easily aggregate. Thus, workability at mixing can be a serious problem. As for the dispersion, when the flammable thermoplastic resin is prepared as an aqueous dispersion, the PTFE dispersion may be easily mixed with the thermoplastic resin. In contrast, when the thermoplastic resin is in the form of powder or pellets, the PTFE dispersion is required to be subjected to a step of removing unnecessary water and surfactants in the dispersion.
There is a requirement for a PTFE dripping inhibitor excellent in handling characteristics of powder. Such inhibitor, however, has another problem. Specifically, before a PTFE dripping inhibitor is added to a raw resin, a PTFE powder and the raw resin need to be premixed. During the premix, the mixed powder of the raw resin and PTFE aggregates to cause blocking in a mixing machine or a feeder of an extruder, which significantly reduces workability and productivity.
In order to solve these problems, Patent Literatures 15 to 17 and the like disclose attempts to improve flame retardance of thermoplastic resin compounds by addition of a mixture of PTFE particles and organic polymer particles (a powder mixture containing PTFE). Patent Literature 15 discloses that the flame retardance can be improved by adding a powder coagulated in a mixture of a polytetrafluoroethylene dispersion liquid and an aromatic vinyl polymer dispersion liquid. Patent Literature 16 discloses that a powder having excellent handling characteristics can be produced by polymerizing an organic monomer in the presence of a polytetrafluoroethylene dispersion liquid. Patent Literature 17 discloses that a thermoplastic resin compound excellent in flame retardance and impact resistance can be formed from a polycarbonate, an acrylonitrile-styrene-butadiene copolymer, and a polyorganosiloxane containing a composite rubber graft copolymer.
However, when a certain amount of a powder mixture containing PTFE is added as a dripping inhibitor to a thermoplastic resin, the powder mixture containing PTFE produces less anti-dripping effect compared with the case of using the same amount of a dripping inhibitor consisting solely of PTFE. This is because the amount of PTFE contributing to anti-dripping ability is less than that of the dripping inhibitor consisting solely of PTFE. Accordingly, more amount of the dripping inhibitor is required to achieve sufficient effect, which raises the cost.
There is another proposal of using a PTFE molding powder (standard specific gravity: 2.13 to 2.23, classified to ASTM D-1457, type IV, VI, or VII) manufactured by suspension polymerization of TFE as an additive agent for a flammable thermoplastic resin (for example, refer to Patent Literature 18).
Patent Literature 19 also proposes a method of producing a PTFE fine powder from a PTFE latex, which has excellent powder flowability and a high apparent density of at least 0.52 g/ml, by coagulation from the PTFE latex which is preliminarily blended with a large amount of a fluorosurfactant.